always use pseudo return regs

[Richard Henderson <rth at redhat dot com>]

I've had this floating about one of my trees for a while...

Since

2000-12-01  Bernd Schmidt  <bernds@redhat.co.uk>

        * combine.c (cant_combine_insn_p): Only disallow insns involving
        hard regs if they are reg-reg moves.
        (try_combine, can_combine_p, combinable_i3pat): Delete old
        SMALL_REGISTER_CLASSES tests.

we've had a harder time optimizing return values, since
we were using hard registers that we refused to combine.

I'd had a test case, that I've since misplaced, where we
failed to generate cmove instructions in relatively trivial
cases.  I couldn't reproduce them this evening while trying
to show an optimization regression from 2.95.  Oh well.

Applied to mainline only.


r~


        * function.c (expand_function_start): Always use a pseudo
        for the function result value.

Index: function.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/function.c,v
retrieving revision 1.270
diff -c -p -d -r1.270 function.c
*** function.c	2001/05/18 07:25:50	1.270
--- function.c	2001/05/23 07:00:21
*************** expand_function_start (subr, parms_have_
*** 6366,6378 ****
    else if (DECL_MODE (DECL_RESULT (subr)) == VOIDmode)
      /* If return mode is void, this decl rtl should not be used.  */
      SET_DECL_RTL (DECL_RESULT (subr), NULL_RTX);
!   else if (parms_have_cleanups
! 	   || current_function_instrument_entry_exit
! 	   || (flag_exceptions && USING_SJLJ_EXCEPTIONS))
      {
!       /* If function will end with cleanup code for parms, compute the
! 	 return values into a pseudo reg, which we will copy into the
! 	 true return register after the cleanups are done.  */
  
        /* In order to figure out what mode to use for the pseudo, we
  	 figure out what the mode of the eventual return register will
--- 6366,6375 ----
    else if (DECL_MODE (DECL_RESULT (subr)) == VOIDmode)
      /* If return mode is void, this decl rtl should not be used.  */
      SET_DECL_RTL (DECL_RESULT (subr), NULL_RTX);
!   else
      {
!       /* Compute the return values into a pseudo reg, which we will copy
! 	 into the true return register after the cleanups are done.  */
  
        /* In order to figure out what mode to use for the pseudo, we
  	 figure out what the mode of the eventual return register will
*************** expand_function_start (subr, parms_have_
*** 6392,6413 ****
        /* Needed because we may need to move this to memory
  	 in case it's a named return value whose address is taken.  */
        DECL_REGISTER (DECL_RESULT (subr)) = 1;
-     }
-   else
-     {
-       /* Scalar, returned in a register.  */
-       SET_DECL_RTL (DECL_RESULT (subr),
- 		    hard_function_value (TREE_TYPE (DECL_RESULT (subr)), 
- 					 subr, 1));
- 
-       /* Mark this reg as the function's return value.  */
-       if (GET_CODE (DECL_RTL (DECL_RESULT (subr))) == REG)
- 	{
- 	  REG_FUNCTION_VALUE_P (DECL_RTL (DECL_RESULT (subr))) = 1;
- 	  /* Needed because we may need to move this to memory
- 	     in case it's a named return value whose address is taken.  */
- 	  DECL_REGISTER (DECL_RESULT (subr)) = 1;
- 	}
      }
  
    /* Initialize rtx for parameters and local variables.
--- 6389,6394 ----